According to the Business Process Execution Language for Web Services Specification, if the locationPath argument selects a node set of a size other than one during execution, the standard fault bpws:selectionFailure must be thrown by a compliant implementation.

For example, the count() function shown in the following code does not work if there are multiple entries of product elements under StoreRequest; this causes a selectionFailure fault to be thrown:

count(bpws:getVariableData('inputVariable',
 'payload','/ns2:StoreRequest/ns2:product'))

To make this work, change the syntax to the following:

"count($inputVariable.payload/ns2:product)"